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I read once that Einstein was a napper, and that he had an interesting method of napping. He'd put his keys in his hand and his hand over the side of his chair, and when he'd fallen asleep deeply enough that he'd let the keys drop, the noise of the keys hitting the floor would wake him up. A kind of a mechanically timed nap that can't get into too deep a sleep.
I don't think my keys hitting the floor would wake me up, though. So how about a device that would go on a keychain and could be set by a switch to making an alarm clock sound upon impact after a short fall.